Formality is just a way to express the concentration of a substance in solution, regardless of how that substance exists in that solution.
In your case, you'd calculate formality just like you would molarity. Use potassium nitrate's molar mass to determine how many moles you're dealing with
The solution's formality will be
This is exactly how molarity is calculated. However, you wouldn't say that your potassium nitrate solution has a molarity of 0.03 M because you don't actually have potassium nitrate in that solution.
What you have are potassium cations ,
So, when you say you have a 1 M solution of sodium chloride, you actually have a 1 F solution of
By comparison, if you have a 1 F glucose solution, you also have a 1 M glucose solution, because glucose does not dissociate in aqueous solution.